Marihuana (delta 9-THC) has been reported to produce hypothermia in many species, including man. Experiments are described which will elucidate the cerebral site of action of delta 9-THC induced hypothermia. An hypothesis which may elucidate the mechanism of action of the tetrahydrocannabinols at the molecular level is advanced. In vivo and in vitro experiments have been designed to test this hypothesis. The cardiovascular actions of the delta 9-THC appear to arise through a reduction in central sympathetic outflow. Experiments are described to provide information about involvement of the sympathetic system by direct measurement of nerve traffic. It has been established that tolerance develops to some actions of the cannabinols during chronic administration. Experiments are described which will determine whether tolerance can occur at the neuronal and cellular levels. We will determine whether tolerance develops differentially in specific homeostatic systems.